Multichannel communication system



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.IIII'IIIII I I INFN@ I I ESTR Patented June 1, 1954 2,689,153

f.-1nN1-+12ED TIA-Tess faHPA-,-E-:NT YoFiiieE 2,680,153l'MULTI"CHA-NNEIl-*COMM'UNIGATION:ir SYSTEM Wilson P.4 Boothroyd,Huntingdon.- Valley-,wand ,Edgan M.- Creamer, Jr.',f..Phi.ldelphia,Pamesvsignofrs .156. Bhilo .Corporationg Philadelphia; Ba., a,eorporation 'ofjPenlisylx/nia. aikpp'licatio. January 14;:11949;-Seria,1No.z.7.0;953

j. "12,v Claims. .(,CL 179715) The present invention relates tomultiechavnnel f" Tcommunica.tion.. systems-- of :theepulseeemplitude-...az-.modulation type. n

The present application discloses subeltmat- .friterf'which is;describedlend claimed in` aeopend- :;fingfUnited.Statespaten ,f .outmajor..;a1terations. in conjunction. with; .either typeraf-modulating..apparatus fiAtliQughne.of,.theprineipal..features ofthetime interval. The rst of theiabove .classes is 20iequency-.divisionmultiplexing Av.method is; its v-vcornmonlyknownas'ffrequency-divisionffmulti .elativelyenarrowbandwidth,Ineveitlvieless it. has plexi Whilerthe'latter'srsimilarly referred. .toas .een found .that an amplitudefmodulatedfmulti- .f time-divisionmultiplex. leigingrfsystem can.. be.A devised.. .which not'only TheSO-Cilledf'fflequenyzditlsim. .multiplex.xrfquals:thefrequency-division.method .with vresystemIof-confununica/ion islwdelylemployed, for 25e-speci; to; bandwidth..economy, .butwhchinj 'addif tion-.possesses adequate linearity`response' This d, .operates ,over a tions or telegraph-messages .rarerto... love.A oamied over af'single :a,b1e.-.It is--a1s.0.suitab1e.;.foruse where suchsigna'lseare to be transmitted .by,-..Cern tain types of`radio 1'relay.networks.-.:- I nr.- latter application, it has theadvantage. o ipeleting with sieclesimbly'flovin,:bandwidthn .Atl thesame y stime; itfpossessesfthd-iSadVantage,.initspresent .Yyrequencyfmodulatea..carrier form of being unable to load many.,.otheif,types 011. It hasalso beeniound offrelay equipments to. .theirOmaXi-mum.eapaci;y, 35;..lthat 511911.35; @mphtude.modu1ated n systemprotz'Ihis.` is .especially true ANI/Thel.-ethrsef:1312x315.. are videsanadequate sgnal-.to-.noisejratio and subdeSgnedprmarily-fon-.the.transmission 0f,..te1estantial1y..minimizes crosstalkbetweenphannels VSOII 01' 'other WdQ-bandsignalsin .eomparisomwithlather...tme-divisionfmulti- The .time-division:multiplex `system.."o,...oom

withf-thefmanner in which .the ..pu1ses.are..nodu j:latedff.Thesefsubfelasss generally.; include. (a) 'i i Varying the..pulseiamplitude' .and (b.) maryngl the :1 -:pulse ;position,11thatis,changing then-time of; oef 'currence of either.. `;thezleaiding.Qrf-ztrailing. ...eclge of 1thefpulse; oriboth.

i"Theflfpulse-aimplitudeemodulaton ,metho in which the various1intelligence;signalafrchainnels its use'"`The"pulsemosi-tion#modulatioiwiiietlf-4 substantially all of the audioinformation inthe wave is present in a series of samples of the wavetaken at an 8 kilocycle rate.

This sampling principle has been utilized in designing thepulse-amplitude-modulated timesharing-multiplex system described in thepresent application. In one embodiment of the invention, samples from aplurality of audio-frequency channels are combined into an asymmetricalcompositesignal comprising a train of amplitude-modulated pulses. rangednot only to give adequate linearity, but in addition to be readilyadaptable for use either in connection with high-frequency relayingapparatus or with relay networks employing pulse transmission only.Furthermore, the multiplexed signal may frequency-modulate a carrierwave directly, or may modulate a sub-carrier wave In'a physicalembodiment of the system to be described, thirty separate andindependent audio frequency channels are time-multiplexed into a 150kilocycle frequency band. One of these channels transmits an indexingtone for synchronizing purposes, and is also used as an order line. Theremaining twenty-nine channe s are lavailable for any desired form ofaudible communication, such as ordinary telephone conversation, or fortelegraphy.

Each audio channel is designed with a frequency passband of between 300and 3,300 cycles per second, and thus has 'an audio fidelitycorresponding to that of a typical telephone system. 'The passband ofthe order line is from 300 to 2,500 cycles per second, with the indexing(or synchronizing) signal occupying a portion of the y;

remaining space in this channel. The thirty audio signals respectivelyoccupying the thirty audio frequency channels are combined into a pulseamplitude modulated time multiplexed composite signal, which may then beapplied either to modulate the carrier wave of a transmitter, or elsesent out directly over a single cable. At the receiver, the compositemultiplexed signal is resolved into its audio-frequency components.Inasmuch requires a bandwidth of only approximately 150 kilocycles, orless, it compares favorably in this respect with any other known systemof multichannel communication of either the frequencydivision or thetime-division species.

One object of the present invention, therefore, is to provide animproved intelligence-communication system of thepulse-amplitude-modulation of the invention is to provide a thepulse-amplitudetype.

Another object communication system of modulation type quired fortransmission is approximately equal to the normal spectrum of theintelligence signal.

A further object of the invention is a multiplex signaling system whichis suitable for use in conjunction with various types of radio relaynetworks, such as those designed for pulse transmission alone or thosedesigned principally for the relaying of television and other widebandsignals.

A still further object of the invention is to provide an improved formof multi-channel communication system in which an adequatesignalto-noise ratio is maintained, and in which crosstalk betweenchannels is reduced to a minimum. Other objects and features of theinvention will be apparent from the following description of a preferredembodiment and from the drawings, in which:

This system is aras the thirty channel system to provide in which thefrequency band re- 4 Fig. l is a block diagram of a preferred form ofmultiplex communication transmitter system in of multiplex receivingsystem in accordance with the present invention;

Fig. 3a. is a circuit diagram of one form of modulator included in thetransmitting system of Fig. l;

Fig. 3b is a set of waveforms helpful in explaining the operation of themodulator of Fig. 3a;

Fig. 3c illustrates the general characteristics of one form ofband-limiting network in Fig. 1;

Fig. 3d illustrates the circuit of a preferred type of filter which isadapted to perform the function of the band-limiting network of Fig. 3c;

Fig. 3e is a graph of loss vs. frequency for one particular type of sucha band-limiting network;

Fig. e is a set of idealized waveforms which are helpful in explainingthe operation of the filter of Fig. 3d;

Fig. 5 illustrates the circuit details of one form of correction networkincluded in the receiving system of Fig. 2;

Fig. 6 illustrates graphically in an idealized manner the operation ofthe crosstalk-correcting network of Fig. 5;

Fig. '7 is a block diagram of the timing generator Fig.

Fig. 9 is a set of waveforms which in explaining the operation of thesystem of Fig. 8;

Fig. l0 is a block diagram of two of the Ichannel separators included inthe receiver of Fig. 2;

Fig. ll illustrates the circuit components of Fig. 10;

Fig. 2a illustrates the response characteristic of another band-limitingnetwork of the type shown in Fig. 3c;

Fig. 12b illustrates the approximate relative amounts of crosstalkpresent in adjacent channels when employing a filter having the responsecharacteristic of Fig. 12a;

Fig. 13 illustrates a preferred form of multichannel correction networkdesigned to eliminate the crosstalk shown in Fig. 12b;

Fig. 14a. is a graph showing possible channel signal voltages, at timeintervals equal to the channel intervals, for a nlter having thecharacteristics of Fig. 3e;

Fig. llb is a block diagram of a corrector network for substantiallyeliminating the crosstalk shown in Fig. 14a;

Fig. 14o is a table of output voltages from the corrector network ofFig. 14h at the time intervals shown in Fig. 14a; and

Figs. 15a. and 15b are graphs of amplitude vs. frequency and amplitudevs. time, respectively, for a modified form of the filter network ofFig. 3d.

In accordance with a principal feature of the present invention, eachintelligence channel is sampled at a rate dependent upon the highestintelligence frequency contained in the signal in that channel. Thissampling process detects the instantaneous amplitude of the signal ineach channel, and, by sampling the channels in sequence, the channelinformation is made available for interleaving into a compositemultiplexed signal.

In a preferred embodiment, the apparatus for carrying out the aboveprocess includes a pulse generator feeding an artificial delay line. Thepulse fromv the generator is preferably of triinput` pulses. the delayline, therefore,` properly-timed sampling amplitude-modulate orfrequency-modulate acarrier Wave for transmission by any suitable formof translating device.

It has been found that the bandwidth neces-- sary for the satisfactoryreproduction of the intelligence contained in each of thesignal'channels may be minimized Without e a rapid attenuationthereafter so as to be down 40 db at 260 kilocycles.-

purpose of-`sampling Such an attenuation Inasrnuoh: asl the: preseamplitude. modulationi `it* erosstalk.

It hasr also been of the; higher frequencies.. however; clauses.:v

nt. disclosure: employs is. necessary that; at

reference wave, having a frequency'- equal to' the ceiver to provide areference level', or base,l on

which the amplitude-modulated intelligence signal maybe superimposed.

modulation' of the signal to complished without crosst rIih-ispermitsthede'- bemore'readi-ly acalk. It has also been band`frequency-division system.

Transmitter Referringnow to Fig-.- 1 is shown a schematic blo ferredform ofpulse-ampli of the drawings, there ck diagram of apretude-rnodulated multiv piex transmitting system in accordance withthe angular pulses `having a constant'repetition rate.

While the pulse `generator yable type know-n in the ar I c may lbeof-any suitt; one particularly ap- .is incorporated therein.

lproduce satisfactory results,

propriete design is described and claimed in the copending applicationSerial No. 14,691, referred to above.

The repetition rate of the pulses produced by the generator I is 8kilocycles. In other words, the' peak of each pulse is spaced in timefrom the peak of the immediately preceding pulse by an interval of 125microseconds. Furthermore, for reasons which will later becomeapp-arent, each triangular pulse has an effective width at its basewhich is no greater than 8 microseconds. These pulses from the generatorI0, which may have a waveform such as shown in the drawing by thereference numeral I2, are applied to the input terminal of a delaynetwork I4. This network I4 may be or any form known in the art,

such, for example, as a plurality of series-connected inductors andshunt-connected capacitors arranged to form individual sections orunits. Network Ill is provided with 30 equally-spaced output taps chosenso that the time delay for each section of the network is approximately.4.16 microseconds. The total delay interval for .the entire network isthus 4.16-30, or 125 microseconds, and is substantially equal to theperiod of the pulses I2. Such delay networks are known in the art, butone type of delay network which is particularly suited for this purposeis shown inthe copending application Serial No. 14,691.

In order that the delay network I4 may remove the high-frequencycomponents present in the pulse output ofl the generator I0, a low-passfilter This filter is designed to have a cut-oil frequency of, forexample, 200 kilocycles. lt may take the form of a number of extra L-Csections located at the input end of the delay network. Accordingly, thepulses I2, which appear successively at the output taps #tI-#30 or thenetwork I4, have a waveform in which the sharp peak of each pulse isrounded off, as shown by the reference numeral It.

The characteristic impedance of the delay network I4 is of coursedetermined by the particular values o the inductors and capacitorsmaking up the assembly. It has been found in practice that acharacteristic impedance of 2500 ohms will and a suitable terminatingimpedance of this value is used.

One important characteristic of the delay network I4 is that it does notintroduce any appreciable change in the waveform of the pulses I6 asthey travel therealong. After the output pulses I2 from the generator I0have passed through the first few sections of the delay network I4'(which constitute the low-pass filter), and have arrived at the firstoutput tap of the network with the shape shown at I6, no significantchange occurs in the waveform of the pulses until after they pass thelast output terminal #30.

summarizing the above, a pulse I2 applied to the input terminal of thenetwork I4 appears at the output taps til-#30 with the waveform I6successively at times spaced approximately 4.16 microsecondsapart. Thewave retains substantially this same shape at each output terminal ofthe network.

As previously mentioned, the transmitting system of Fig. l is designedto multiplex thirty audio channels on a time-sharing basis. This isaccomplished by sampling the intelligence signal in each channel at arate equal to at least twice the highest frequency contained therein.This sampling process detects the instantaneous amplitude of theintelligence signal at the instant when sampling occurs, and, since thechannels are sampled in sequence, the channel information is availablefor intermixing into a composite multiplexed signal.

As the pulse IE passes the various output taps of the network I4 insequence, it becomes a timing wave for the purpose of sampling therespective intelligence channels. The embodiment of the inventionillustrated includes thirty such channels, although this number wasarbitrarily chosen and thus is merely exemplary. One o these channelstransmits an indexing tone for synchronizing purposes and is also usedas an order line. The remaining twenty-nine channels are available foraudio communication.

Each of the thirty output taps or terminals of the delay network I4 isconnected to one of thirty modulators I8. Twenty-nine of thesemodulators also receive signals from twenty-nine audio input channels,each of which includes a microphone 20 or other source of audiofrequency signals. In order that all frequencies outside the 300 to3,300 cycle range may be eliminated from the output of the microphones20, a filter 22 is provided in each audio channel. The output of eachlter 22, therefore, is an audio signal having no frequency higher thanapproximately 3,300 cycles per second.

Each of these audio signals is applied to its respective modulator I8,which also receives a timing signal, in the manner above described, fromone output tap on the delay network I4. Inasrnuch as the highest audiofrequency is limited by the filters 22 to a value of approximately 3,300cycles per second, it will be seen that the 8 kilocycle wave It willsample the audio information in each channel at a rate equal to at leasttwice the highest audio frequency. Furthermore, the amplitude o the 8kilocycle energy appearing at the output of any particular one of themodulators I8 will depend upon the instantaneous value of the audiosignal applied to that particular modulator at the instant when asampling pulse is also applied thereto. Thus the signal in each audiochannel may be transmitted without any appreciable loss of theinformation contained therein.

The single remaining modulator #21 receives both an indexing tone at afrequency of 3,900 cycles from a generator 24 and also the output of anorder line filter 26. Inasmuch as the order line information in theembodiment described doesnot require as high a frequency range as that othe remaining audio inputs, the order line filter 26 (which is connectedto a microphone 28), has a frequency passband of from 300 to 2,500cycles. Since the highest frequency applied to the indexing tone a-ndorder line modulator is 3,900 cycles per second, the intelligence inchannel #21 will still be sampled at least twice per cycle by the 8kilocycle timing wave I6. y

Although any one of the modulators I8 might have been selected toreceive the combined output of the indexing tone generator 24 and theorder line filter 26, in the present embodiment channel #21 was selectedfor this purpose. Thus, each one oi the thirty modulators I8 isconnected to receive a triggering pulse in timed sequence from one ofthe thirty taps on the delay network I4.

The respective outputs of the modulators I8, representing thirtychannels of amplitudemodulated pulses, are then combined into a singlemultiplexed signal by the combining circuit 30. The signal in the outputof the circuit 30 may -havefawaveformsuchaserepresentedibyrtherefference numeral "32. This 'wave "32'is a composite multiplexed signalcomposed of |thirty phasefdelayed ypulsesderived v'from each `one of the8 `kilocycle timing pulses it`,:or 240,000 amplitude-V `modulated4pulses 'per second. *Each 'thirtieth Apulse in this wave representstheintelligence of yone-particular channel.

"One of 'the principal features --o'f the Vpresent invention resides inthe 'abilityof the disclosed 'According tothis feature ofthe-'presentinvenl Ltion, therefore, a vtransmission 'bandwidth ofV onlyapproximately 150 llkilocycles is lrequired. A1-

-frequency-doubling circuit.

f'In 'order that f the multiplexed signal may lbe transmitted withinthe-above-'mentioned 150i kilocycle pa-ssband, the composite-signalS isapplied Fharmonics -(up `to tat least the 3ifteen'th') and,Vvl"turthermore, passes :the ztwo sidebands :of each @of `:these`harmonics fwith -substantially .equal amplitude. However, it isrecognized that the cut-on of the bandl-imiting network 34, such asshown by the response curve 3E in Fig. 1, will *introduce lconsiderablecrosstalkinto the signal vv'3l-'unless' it Iis compensa-tedffor. Thevmeans for producing such 'a compensation are an vessential Aportion 'ofA4the invention, and "will 'be fully idescrlibed inl connection withAaw'description ofthe =receiving-apparatus;Jas set .for-th below. 1A4portion offtheoutput fof -the oscillator Bis 35i-'of ccnstantampltude.Any necessary phasing fingaunitdil. Theoutputflof vthe-united iscomvbined-in :properly timedv relation with the output of:thefba-nd-limiting network 34; .and the resultwave isfemployed tomodulate either a trans- Amitter '1112 lora-11151 other `type sofv.translatingl device.

Receiver In"Fig.2'is illustrated a block diagram of one :form ofmultiplex vreceiving vsystem in accordance with the present invention.`V'lhefreceiving system of Fig. 2 is particularly 'suited 'to reproducethe intelligence present ina multiplexed signal tra-nsmitted by a systemsuch as illustratedin Fig. 1.

transducer so that. the lntelligence ,conta-inedin the signal may bereproduced.

amplitude-modulated pulses .having-a waveform .similar to .thatrepresent-ing the'combmed outputs -of the l.band-.limiting network .'34andthe phasing unit .-40 yof the-..transmitter illustrate in Fig, l.This `out-put from vthe treceiver 56,

rhowever, .is .not truly representative of the multiplexed intelligence:signal output .of thel combining circuit `llin Fig. due-to -the factthat, as

.the transmitted .signal 'by the cut-oI-of the -lter incorporated-fintheflimiting .network 34. -Consequently, the Areceiver ofdig. 2-includesarcor- .rection network V52 to Vwhichfthe output ofthe Vvreceiver .50vis applied.

The filter network Yslfof Fig. --1 A`preferably has a responsecharacteristic which, -iwhilevsloping onlyfslightlyout toa frequency Aofapproximately .150kilocycles, nevertheless iswnot completely'flat overthisportion .of the spectrum. Although it may be .down vonly`approximately 8..db'lat vv150 Ifki-locycles, Veven this relativelyYslight slope enough to -produce crosstalk between adjacentintelligenceehannele .I-f :some v`means vwere not -an-yAsuch-crosstallrwhichmay exist is reduced :to a- 4negligible value.

'In one form, the correction network 52 inance. It ytherefore Vproducesreflections, the

v cfV that particular pulserema-iningat the input ,terminals at the,precise instant-of larrival ofl the pulse representing the nextsucceeding channel. Hence, the only signal effectively present at thetime of arrival of a following pulse is that which is actually presentin the latter pulse itself, and no residual or carry-over voltageremains from the pulse which preceded it. A second delay line isemployed to compensate for crosstalk introduced from the energy in theimmediately following channel. A complete description of the details ofthe correction network 52 will be given in connection with adescription` of Fig. 5, and it is believed that the above is suicient atthis point to provide an understanding of the function of thisparticular component in the receiver system.

It will be appreciated from the above description that the correctionnetwork 52 acts to reduce crosstalk between adjacent channels at oneprecise instant in each cycle when the residual voltage of a particularpulse is cancelled by the presence of the equal and opposite voltagederived by reflection. However, it will also be clear that for eachchannel this cancellation occurs at only one instant. Hence, in order toderive a signal representative of the actual intelligence present in thereceived wave, it is necessary that the various channels be sampled atthe exact moments when such crosstalk cancellations occur. It is forthis purpose that the 120 kilocycle wave output of the phasing unit 4Bin Fig. 1 was combined with the output of Y the band-limiting network34.

Referring again to Fig. 2, there is provided a lter 54 which isconnected as shown to the receiver so as to produce a 120 kilocycleenergy wave bearing a timed relation to the received signal. 'Ihis 120kilocycle energy from filter 54, which is unmodulated, is then passedthrough a frequency doubler 5S and a phasing unit 58 to produce a 240kilocycle wave of constant amplitude, part of which is mixed with theintelligence signal output of the corrector network 52 in an amplifierand cathode follower 59. The phasing unit 58 should be adjusted so thatthe peaks of the 240 kilocyclewave occur at the precise instants whenthe correction network 52 reduces the crosstalk "between the adjacentintelligence channels substantially to zero. In other words, units 54,56 and 58 act to provide a base, or pedestal, upon which theamplitudemodulated multiplexed signal rection network 52 may besuperimposed. This multiplexed signal, which now possesses a referenceor base voltage, is applied simultaneously over the conductor E0 to eachone of thirty channel separators 62.

The receiver of Fig. 2 also includes a timing generator 64 which hasfunctions similar to that of the pulse generator I (l in combinationwith the delay network I4 in Fig. l. That is, the generator 64 providesa timing wave which is produced by pulses of an 8 kilocycle repetitionfrequency traversing a delay line having thirty equally-spaced outputtaps. The delay period between the successive output taps is identicalto that provided by the delay network i4 in Fig. l or, in other words,about 4.16 microseconds. The details of this timing generator Bd will beset forth in connection with a description of Figs. 7, 8 and 9, and itwill merely be stated at this time that the generator 64 receives both asynchronizing voltage from an indexing tone filter 58 over a conductorS1, and also a portion of the unmodulated 240 kilocycle output of thephasing unit 58 over a conductor 68.

The thirty channel separators 52, including output of the cortheindexing tone and order line channel separator #21, are all suppliedwith the intelligence signal from the amplifier 59, and also with timingpulses from the generator 64. These latter pulses gate the channelseparators 62 in such a manner that there is no output from the latterexcept during the occurrence of a timing pulse. However, when such atiming pulse. does occur, then the output voltage from the particularseparator `62 to which it is applied rises to a peak value correspondingto the amplitude of the pulse included in the intelligence signal at theinstant of triggerin Each of the channel separators 62 thus in effectselects one particular channel from the composite multiplexed signal. Inorder that sufficient power be available which is truly representativeof the intelligence in that particular signal channel, it is desirablethat the output wave from each separator be maintained at theintelligence signal level for a sufficient period of time to provideadequate energy for the reproducing apparatus. Accordingly, each of thechannel separators 62 is so arranged that the timing pulse from thegenerator 64 acts to initiate a voltage variation which remains at aconstant level for an appreciable period of time, and is then returnedto its original value by an action of a discharge, or restoring, voltagederived from the immediately preceding channel. In the embodimentillustrated, the voltage in each of the channel separators which isrepresentative of the intelligence information is caused to remainconstant for a time interval of about microseconds, this being slightlyless than the microsecond period of the 8 kilocycle timing pulses.Further details of the channel separators 62 will be given in connectionwith a description of Figs. 10 and Il.

The output of channel separator #2 is applied not only to the indexingtone filter B6 to permit separation of the 3,900 cycle synchronizingwave, but also to a low pass (300 to 2,500 cycle) filter 10 to providethe order line intelligence picked up by the microphone 23 at thetransmitter. The remaining twenty-nine channel separators are connectedto twenty-nine audio filters l2, the respective outputs of which arereproduced in the twenty-nine output circuits thereof, here representedby twenty-nine audio reproducers 14.

Modulators In Fig. 3a is shown a preferred type of circuit foraccomplishing the function of each individual modulator I8 in Fig. 1. Itwill be appreciated that it is the purpose of each such modulator i8 toact as a gating circuit which effectively connects the output of itsrespective audio filter 22 to the modulator output circuit (combiningcircuit 30) in accordance with the application to the modulator of oneof the timing pulses I6 from the delay network I4. The outputs of therespective modulators are then consolidated as shown in Fig. 1 in thecombining circuit 30. It will be further appreciated that the modulatorI8, or gating circuit, must be closed to the output of its associatedaudio filter 22 at all times except when it is opened by the applicationthereto of one of the timing pulses i6 from the delay network.

Accordingly, Fig. 1 may include a pentode 16 (as shown in Fig. 3a) tothe control grid 18 of which the output of an audio lter122 is applied.The screen each one of the modulators I8 in

